1. Field of the Invention
The present invention relates to the use of catalysts for improving the service life of valve regulated lead acid (VRLA) cells, and to a catalyst device which may also be applicable to other types of batteries where it is desirable to recombine excess oxygen with hydrogen produced in the battery cell. More particularly, the present invention relates to improved catalyst devices that minimize catalyst poisoning and control the temperature of the catalytic reaction.
2. Background of the Invention
Significant improvements to VRLA battery cells can be made with the addition of a catalyst for recombining oxygen and hydrogen gasses within the cells as disclosed in U.S. application Ser. No. 09/022,336 filed Feb. 11, 1998, and which is hereby incorporated by reference (see related PCT publication WO 99/41798). One preferred method of adding catalyst to VRLA cells is to couple the catalyst with the pressure relief valve, sometimes referred to as a pressure relief vent or pressure relief vent cap, which provides a convenient means of retrofitting existing cells as well adding catalyst to new cells. A novel catalyst device for use with pressure relief vents is disclosed in U.S. application Ser. No. 09/022,336. While the addition of catalyst provides significant improvement in the performance and of life of VRLA cells, additional improvements to the catalyst itself, particularly with the structures and devices that support or house the catalyst, are believed necessary to gain the full benefits of the catalyst.
High rates of gassing in VRLA battery cells can be caused by conditions such as thermal runaway, malfunctioning power rectifiers, boost charging, charging at abnormally high rates of voltage, and other reasons known in the art. In lead-acid battery cells, oxygen and hydrogen gas is generated due to the electrolysis of water. If a catalyst were installed in a cell experiencing a high rate of gas production, the catalyst would catalyze the oxygen and hydrogen to water in a highly exothermic reaction. The resulting enthalpy could generate enough heat to melt many commonly used plastic materials that compose the battery cover, jar or vent cap assemblies. This high reaction temperature can also limit the materials that can be used to house the active catalytic material.
One catalyst device as described in the above referenced U.S. application Ser. No. 09/022,336 patent application, uses a porous ceramic cartridge to house the catalytic active material. This porous cartridge allows gas to enter the chamber and water vapor to exit the chamber, and prevents the passing of a flame from inside the chamber to outside the chamber if one were to develop due to ignition of the gases. The temperature achieved, as a result of the exothermic reaction, is governed mainly by the amount of gas produced by the battery. In the normal functioning of a VRLA battery, the temperature of reaction is generally not more than 5° to 10° F. above ambient temperature. However, in the case of high rates of gassing, a higher than normal temperature could occur which could lead to melting of the plastic that supports the catalyst cartridge, that composes the vent cap or that composes the cell jar or cell cover. The high temperature could also deform the pressure relief valve that is present in the vent cap for these VRLA cells. A deformed pressure relief valve will change the operating characteristics of the cell. Temperature high enough to melt the plastics used to compose batteries must be avoided.
Another problem found to plague lead acid cells is catalyst poisoning. Precious metal catalysts, such as palladium and platinum are susceptible to poisoning by many chemicals that can limit and negate the effectiveness of the catalysts. Catalyst poisons can be generated in or found in the battery cells. VRLA batteries are constructed of materials that may exhaust gaseous compounds that are poisonous to catalysts. Such poisons include organic compounds such as phthalates used as a plasticizer in PVC battery jars and covers, and inorganic compounds such as sulfides, mercaptins, amines, stibine and phosphates whose base chemicals can exist as an impurity in battery plates and exhaust as the plates corrode. Amines and mercaptins are used extensively as curing compounds in multi-part epoxies which are used in the manufacture of cells by many manufacturers. It is also believed that localized areas of high current densities may cause hydrogen sulfide to form within the cells. Hydrogen sulfide may also be a byproduct of the dry charging process that traps sulfur species in the plates which is then released early in the battery's life. It is believed that compounds such as those described above that have a (−2) state of charge can bond with catalyst activation sites rendering the catalyst effective. The catalyst can also be poisoned by large chain molecules, such as phthalates, that coat the catalyst substrate and then solidifying on it, creating a barrier that prevent the gasses from reaching the catalyst reaction sites. Hydrogen sulfide, SOx species, and other sulfides will also poison precious metal catalysts.
It is further believed that simpler and lower cost catalyst devices are desirable.
Accordingly, one object of the present invention is to limit the temperature of the catalytic reaction.
Another object of the present invention is to prevent poisoning of the catalyst.
Another object is to lower the manufacturing costs of catalyst devices.